Gaia news portal

Gaia news portal

The news in this bulletin board is compiled from various sources across the world wide web.

02/10/2015: The colour of Gaia's eyes
From the Gaia Image of the Week page: Gaia's CCD detectors are covered with a thin film. To optimise the transmission of light from the vacuum of space into the silicon of the light detectors, the silicon is covered with a very thin anti-reflective (AR) coating. The thickness chosen for that layer depends on the wavelength range which is to be observed with a particular CCD detector: for the CCDs of Gaia's "Blue Spectrometer" (BP), designed to obtain spectra of stars in the short-wavelength, blue to ultraviolet part of the spectrum, this layer is the thinnest. For the CCDs used in the "Astrometric Field" (AF), designed to determine the position of stars from as much light as possible, all the way from ultraviolet to infrared, the anti-reflective coating is thicker. For the CCDs of Gaia's "Red Photometer" (RP) and "Radial Velocity Spectrometer", both used for producing spectra at longer wavelengths in the red and near-infrared, the film is the thickest. more
24/09/2015: Estimating distances from parallaxes
From the Gaia Image of the Week page: Parallax is defined as the reciprocal of distance. But when we measure a parallax, this measurement is always noisy, and its simple reciprocal is not necessarily a good estimate of the distance. In fact, it has been known for some time that once the fractional parallax error is more than about 20%, the reciprocal parallax is a rather poor and biased estimate of the distance. This is an issue for Gaia, because about 80% of all stars which Gaia observes will have fractional parallax errors larger than 20%. This fact can be seen in the above plots, which both show the cumulative distribution of fractional parallaxes. The black line in the right panel shows the fraction of Gaia stars (vertical axis) which are expected to have a fractional parallax error below the value given on the horizontal axis. This was calculated using the GUMS catalogue and the sky-averaged, post-launch, Gaia astrometric accuracy model assuming five years of observations. The red line is for the actual fractional parallax errors from the Hipparcos mission. Hipparcos generally has a larger fraction of stars with a smaller given fractional error than Gaia, but Hipparcos observed a far smaller absolute number of stars and with larger absolute parallax errors. The left panel shows the same information but on a logarithmic scale. more
25/08/2015: Gaia's first year of scientific observations
From ESA's Gaia site: Last Friday, 21 August, ESA’s billion-star surveyor, Gaia, completed its first year of science observations in its main survey mode. After launch on 19 December 2013 and a six-month long in-orbit commissioning period, the satellite started routine scientific operations on 25 July 2014. Located at the Lagrange point L2, 1.5 million km from Earth, Gaia surveys stars and many other astronomical objects as it spins, observing circular swathes of the sky. By repeatedly measuring the positions of the stars with extraordinary accuracy, Gaia can tease out their distances and motions through the Milky Way galaxy. more
07/08/2015: Demonstrating Gaia's astrometric potential
From the Gaia news page: The Hertzsprung-Russell (HR) diagrams presented here were produced as part of a first validation of the astrometric capabilities of the instrument and of the Basic Angle variation measured by the on-board interferometer (Basic Angle Monitor; BAM). Based on less than one year of data from the routine observation phase of Gaia, they give an exciting hint of what the mission will deliver. more
ArXiv:1507.08066 by G. Kordopatis et al: We investigate, using the Gaia-ESO Survey internal Data-Release 2, the properties of the double sequence of the Milky Way discs (defined chemically as the high-alpha and low-alpha populations), and discuss their compatibility with discs defined by other means such as metallicity, kinematics or positions. This investigation uses two different approaches: in velocity space for stars located in the extended Solar neighbourhood, and in chemical space for stars at different ranges of Galactocentric radii and heights from the plane. The separation we find in velocity space allows us to investigate, in a novel manner, the extent in metallicity of each of the two sequences, identifying them with the two discs, without making any assumption about the shape of their metallicity distribution functions. Then, using the separation in chemical space, we characterise the spatial variation of the slopes of the [alpha/Fe] - [Fe/H] sequences for the thick and thin discs and the way in which the relative proportions of the two discs change across the Galaxy. We find that the thick disc (high-alpha sequence), extends up to [Fe/H]~ +0.2 and the thin disc (low-alpha sequence), at least down to [Fe/H]~ -0.8. Radial and vertical gradients in alpha-abundances are found for the thin disc, with mild spatial variations in its [alpha/Fe] - [Fe/H] paths, whereas for the thick disc we do not detect any such spatial variations. The small variations in the spatial [alpha/Fe] - [Fe/H] paths of the thin disc do not allow us to distinguish between formation models of this structure. On the other hand, the lack of radial gradients and [alpha/Fe] - [Fe/H] variations for the thick disc indicate that the mechanism responsible for the mixing of the metals in the young Galaxy (e.g. radial stellar migration or turbulent gaseous disc) was more efficient before the (present) thin disc started forming. more
31/07/2015: Asteroids all around
From the Gaia Image of the Week page: Gaia observes more than a billion stars on the whole sky, without knowing in advance where they are. However, as each source is observed multiple times, the Initial Data Processing (IDT, a highly sophisticated piece of software running on the data transmitted by the satellite, developed by the University of Barcelona team) has the task of grouping together multiple observations of the same source. This task, the so-called "cross-matching", involves comparing the positions recorded by Gaia. If two sources are observed, within the uncertainty, at the same position on the sky they are recognized to be – in fact – the same source. For asteroids, this cannot work, as they are always moving amongst the stars - slowly (typically, an asteroid in the Main Belt can take a couple of days to move a distance of a Moon diameter) but fast enough for Gaia (several pixels during a single transit on the focal plane)! As a result, Gaia never sees an asteroid at the same place, and the cross-matching described above leaves these detections as "orphans" that do not repeat over time. more
17/07/2015: Gaia satellite and amateur astronomers spot one in a billion star
From the Gaia Image of the Week page: An international team of researchers, with the assistance of amateur astronomers, have discovered a unique binary star system: the first known such system where one star completely eclipses the other. It is a type of two-star system known as a Cataclysmic Variable, where one super dense white dwarf star is stealing gas from its companion star, effectively ``cannibalising'' it. more
arXiv:1507.04353 by T. Antoja et al: We present a method to identify Ultra Faint Dwarf Galaxy (UFDG) candidates in the halo of the Milky Way using the future Gaia catalogue and we explore its detection limits and completeness. The method is based on the Wavelet Transform and searches for over-densities in the combined space of sky coordinates and proper motions, using kinematics in the search for the first time. We test the method with a Gaia mock catalogue that has the Gaia Universe Model Snapshot (GUMS) as a background, and use a library of around 30 000 UFDGs simulated as Plummer spheres with a single stellar population. For the UFDGs we use a wide range of structural and orbital parameters that go beyond the range spanned by real systems, where some UFDGs may remain undetected. We characterize the detection limits as function of the number of observable stars by Gaia in the UFDGs with respect to that of the background and their apparent sizes in the sky and proper motion planes. We find that the addition of proper motions in the search improves considerably the detections compared to a photometric survey at the same magnitude limit. Our experiments suggest that Gaia will be able to detect UFDGs that are similar to some of the known UFDGs even if the limit of Gaia is around 2 magnitudes brighter than that of SDSS, with the advantage of having a full-sky catalogue. We also see that Gaia could even find some UFDGs that have lower surface brightness than the SDSS limit. more
arXiv:1507.02963 by D. Michalik et al: Gaia's astrometric solution aims to determine at least five parameters for each star, together with appropriate estimates of their uncertainties and correlations. This requires at least five distinct observations per star. In the early data reductions the number of observations may be insufficient for a five-parameter solution, and even after the full mission many stars will remain under-observed, including faint stars at the detection limit and transient objects. In such cases it is reasonable to determine only the two position parameters. Their formal uncertainties would however grossly underestimate the actual errors, due to the neglected parallax and proper motion. We aim to develop a recipe to calculate sensible formal uncertainties that can be used in all cases of under-observed stars. Prior information about the typical ranges of stellar parallaxes and proper motions is incorporated in the astrometric solution by means of Bayes' rule. Numerical simulations based on the Gaia Universe Model Snapshot (GUMS) are used to investigate how the prior influences the actual errors and formal uncertainties when different amounts of Gaia observations are available. We develop a criterion for the optimum choice of priors, apply it to a wide range of cases, and derive a global approximation of the optimum prior as a function of magnitude and galactic coordinates. The feasibility of the Bayesian approach is demonstrated through global astrometric solutions of simulated Gaia observations. With an appropriate prior it is possible to derive sensible positions with realistic error estimates for any number of available observations. Even though this recipe works also for well-observed stars it should not be used where a good five-parameter astrometric solution can be obtained without a prior. Parallaxes and proper motions from a solution using priors are always biased and should not be used. more
arXiv:1507.02105 by C.A.L. Bailer-Jones: Astrometric surveys such as Gaia and LSST will measure parallaxes for hundreds of millions of stars. Yet they will not measure a single distance. Rather, a distance must be estimated from a parallax. In this didactic article, I show that doing this is not trivial once the fractional parallax error is larger than about 20%, which will be the case for about 80% of stars in the Gaia catalogue. Estimating distances is an inference problem in which the use of prior assumptions is unavoidable. I investigate the properties and performance of various priors and examine their implications. A supposed uninformative uniform prior in distance is shown to give very poor distance estimates (large bias and variance). Any prior with a sharp cut-off at some distance has similar problems. The choice of prior depends on the information one has available - and is willing to use - concerning, for example, the survey and the Galaxy. I demonstrate that a simple prior which decreases asymptotically to zero at infinite distance has good performance, accommodates non-positive parallaxes, and does not require a bias correction. more
03/07/2015: Counting stars with Gaia
From the Gaia Image of the Week page: As Gaia scans the sky to measure positions and velocities of a billion stars with unprecedented accuracy, for some stars it also determines their speed across the camera's sensor. This information is used in real time by the attitude and orbit control system to ensure the satellite's orientation is maintained with the desired precision. more
28/05/2015: Short period/faint magnitude Cepheids in the Large Magellanic Cloud observed by Gaia
From the Gaia Image of the Week page: In December 2014 a first reduction of the photometry acquired by Gaia during 28 days of Ecliptic Pole Scanning Law (EPSL) and 3 days of Nominal Scanning Law (NSL) was delivered to CU7, the Coordination Unit in charge of performing the analysis of the variable sources observed by Gaia. In the week of 19-25 March 2015, the full chain of the CU7 pipeline ran on the EPSL+NSL dataset (about 800,000 sources which have more than 20 Field-of-View transits), starting from the general Variability Detection, general Characterization, proceeding through the global Classification and ending with the detailed checks and typecasting of the Specific Objects Study (SOS). The time-series photometry of 1242 sources in the South Ecliptic Pole (SEP), which covers an external region of the Large Magellanic Cloud (LMC), was fed into the Cepheid & RR Lyrae SOS pipeline, which returned a rich harvest of LMC RR Lyrae stars (more than 800, including some hundreds of new discoveries, with typical average magnitudes around G~19.5 mag and periods in the range of about 0.15  to 1.0 days) and a few Cepheids at the short period/faint end of the LMC Cepheids period-luminosity distribution. Some of them are new Cepheids discovered by Gaia. more
arXiv:1505.07019 by R. J. Jackson et al: The Gaia-ESO Survey (GES) is a large public spectroscopic survey at the European Southern Observatory Very Large Telescope. A key aim is to provide precise radial velocities (RVs) and projected equatorial velocities (v sin i) for representative samples of Galactic stars, that will complement information obtained by the Gaia astrometry satellite. We present an analysis to empirically quantify the size and distribution of uncertainties in RV and v sin i using spectra from repeated exposures of the same stars. We show that the uncertainties vary as simple scaling functions of signal-to-noise ratio (S/N) and v sin i, that the uncertainties become larger with increasing photospheric temperature, but that the dependence on stellar gravity, metallicity and age is weak. The underlying uncertainty distributions have extended tails that are better represented by Student's t-distributions than by normal distributions. Parametrised results are provided, that enable estimates of the RV precision for almost all GES measurements, and estimates of the v sin i precision for stars in young clusters, as a function of S/N, v sin i and stellar temperature. The precision of individual high S/N GES RV measurements is 0.22-0.26 km/s, dependent on instrumental configuration. more
15/04/2015: GBOT embarks on asteroid finding programme
From the Gaia news page: Using telescopes on Earth, Gaia's Ground Based Optical Tracking campaign (GBOT) has been delivering daily datasets, which are used to determine the satellite's position, since launch. Making use of data that are generated for its primary purpose, the tracking of Gaia, the campaign has now found an additional science application: the detection of asteroids. Using the astrometric pipeline, several tens of known and unknown asteroids are being detected every night. Find out more about the asteroid programme here. more
09/04/2015: Gaia honours Einstein by observing his cross
2015 is the International Year of Light and marks an important milestone in the history of physics with the one-hundredth anniversary of Einstein's Theory of General Relativity. Having the ability to test some of its aspects to an unprecedented accuracy, Gaia will probe the tiny deviations predicted by General Relativity in our solar system. However, the satellite will also see other evidence at work such as gravitational lensing shown in the images of the so-called Einstein Cross, discovered in 1985 (Q2237+030), and of a very similar one (HE0435-1223), discovered in 2002. more
24/03/2015: Gaia engineering data release for cosmic ray studies

24/03/2015: Gaia engineering data release for cosmic ray studies

From the Gaia news page: The radiation environment at L2 is of great interest to current and future space missions with operational orbits around the Sun-Earth Lagrange point. End-of-Life performance and transient events on the semiconductor imaging detectors are based on models that need to be correlated to real data. Transient events in the form of cosmic ray trails recorded on the Gaia Focal Plane CCDs may support these studies and a small initial engineering data release has been prepared for this purpose. The complete data set covering data acquisitions from May 2014 to January 2015 and the corresponding technical note are available for download here. A preliminary analysis on radiation effects including cosmic ray events on the Gaia CCDs has been done in a SPIE paper.
05/03/2015: RR Lyrae stars in the Large Magellanic Cloud as seen by Gaia
During the first part of the Gaia mission in Ecliptic Pole Scanning Law, Gaia frequently observed the South Ecliptic Pole (SEP) which covers an external region of the Large Magellanic Cloud (LMC). A couple of hundred RR Lyrae stars are known to populate the central 1 square degree of the Gaia SEP. Their typical average apparent magnitudes are around 19.5 mag in V. The magnitude of RR Lyrae stars is fainter and amplitudes are larger in blue photometric bands. more
26/02/2015: First Gaia BP/RP deblended spectra
To test the photometric deblending software, which is to be fully deployed at the Cambridge Data Processing Centre by the end of 2015, a set of known bright double stars has been processed. The figure above shows the output of the code applied to the BP/RP spectra of the double star "HD 270801", the first Gaia BP/RP deblended object. more
06/01/2015: Close encounters of the stellar kind
Stars are in constant relative motion, and inevitably, some will come close to the Sun. But how close? And what effect (if any) might they have? A sufficiently close encounter could perturb the orbits of comets in our Oort cloud, a primordial reservoir of billions of comets in the outer solar system extending to perhaps 0.5 pc (100 000 AU) from the Sun. Gravitational perturbations could push these comets onto orbits which send them into the inner solar system, where they could in principle collide with the Earth. more
19/12/2014: First Gaia launch anniversary
Exactly one year ago, at 10:12 CET, Soyuz VS06 with Gaia on board took off from Europe's spaceport in French Guiana, ferrying ESA's billion-star surveyor into space. After an exciting year with a successful L2 orbit insertion, a challenging commissioning period and the start of routine operations, Gaia is now scanning the sky, mapping on average 40 million stars a day. On the occasion of the first launch anniversary, six members of the Data Processing and Analysis Consortium (DPAC) share their launch and commissioning memories. more
17/12/2014: Status of Gaia straylight analysis and mitigation actions
During commissioning it was found that the sky background observed by Gaia is significantly higher than expected. In the past months this issue has been investigated by an independent team involving experts from ESA, industry and the Data Processing and Analysis Consortium (DPAC). Following these investigations, it is now clear that the straylight is caused by two sources: the Sun and bright objects in the sky, most notably the Milky Way. more
06/11/2014: New study ups the predicted number of exoplanets discovered by Gaia
In a study to appear in The Astrophysical Journal, and published today on arXiv, it is estimated that Gaia will have discovered some 20,000 Jupiter mass exoplanets by the time it completes its survey in 2019. Former Gaia project scientist Michael Perryman teamed up with Lennart Lindegren from Lund Observatory and Princeton University astronomers Joel Hartman and Gáspár Bakos to make a revised estimate of the expected numbers. more
12/09/2014: Gaia discovers its first supernova
While scanning the sky to measure the positions and movements of stars in our Galaxy, Gaia has discovered its first stellar explosion in another galaxy. This powerful event, now named Gaia14aaa, took place in a distant galaxy some 500 million light-years away, and was revealed via a sudden rise in the galaxy's brightness between two Gaia observations separated by one month. more
29/07/2014: Commissioning review: Gaia ready to start routine operations
The commissioning of Gaia came to its formal end on 18 July 2014 when the board members of the mission in-orbit commissioning review (MIOCR) confirmed the readiness of the space and ground segments to start routine operations. The review summarised the commissioning activities both on ground and in orbit. New scientific performance estimates have been calculated since using in-orbit commissioning data. more
27/06/2014: Commissioning the Radial Velocity Spectrometer
The Radial Velocity Spectrometer (RVS) is one of three instruments onboard Gaia (see Figure 1). It is designed to measure the line-of-sight velocity component of Gaia stars (radial velocity, RV) to complement Gaia astrometry, which measures the transverse velocity component (parallax converts proper motions to transverse velocity). Combining the radial and transverse velocities gives the 3D space velocity of Gaia stars, allowing Gaia to produce not only a map of where Gaia stars are but how they are moving.
16/06/2014: Preliminary analysis of stray light impact and strategies
A series of exhaustive tests have been conducted over the past few months to characterise some anomalies that have been revealed during the commissioning of Gaia following its successful launch in December 2013. more
6/2/2014: Gaia comes into focus
ESA's billion-star surveyor Gaia is slowly being brought into focus. This test image shows a dense cluster of stars in the Large Magellanic Cloud, a satellite galaxy of our Milky Way. Once Gaia starts making routine measurements, it will generate truly enormous amounts of data. To maximise the key science of the mission, only small 'cut-outs' centred on each of the stars it detects will be sent back to Earth for analysis. more
19/12/2013: Liftoff for ESA’s billion-star surveyor
ESA’s Gaia mission blasted off this morning on a Soyuz rocket from Europe’s Spaceport in Kourou, French Guiana, on its exciting mission to study a billion suns. Gaia is destined to create the most accurate map yet of the Milky Way. By making accurate measurements of the positions and motions of 1% of the total population of roughly 100 billion stars, it will answer questions about the origin and evolution of our home Galaxy. more
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